Dosage-dispensing devices are known for dispensing small quantities, for example of toxic substances, with high precision into small target containers. Frequently, such recipient containers are placed on a balance in order to weigh the substance delivered out of the dosage-dispensing device, so that the substance can subsequently be further processed according to given specifications.
The substance to be dispensed is contained for example in a source container which is equipped with a dispensing head. It is desirable to deliver the substance to the outside through a small outlet opening in the dosage-dispensing device, so that the substance can be filled in a targeted stream also into a container with an opening of narrow cross-section.
Dosage-dispensing devices for dry and/or powdery substances of a pourable consistency belong to the known state of the art and are in current use. For example in WO 2004/042334 A1, a dosage-dispensing device is described which includes a drive device and a dispensing head that can be set in place in the drive device. The dispensing head is connected to a source container and has a closable outlet at its underside. When the apparatus is in an operating position, a balance is arranged below the dispensing head, so that target containers can be set on the balance pan.
During the dosage-dispensing process, the substance to be dispensed is moved out of the source container and measured into the target container. The increase in weight which is measured continuously by the balance during the dosage-dispensing process is communicated to a control- and regulation unit, which stops the dispensing process as soon as the desired quantity of substance has been dispensed or when the substance quantity in the target container has reached the target weight as defined by the user.
The attainable precision of the dosage-dispensing device or, more specifically, the precision that can be attained in meeting the target weights within a tolerance band can be limited, because in dispensing processes of this kind the dosage-dispensing device has a tendency to deliver more than the prescribed quantity of substance. The reason for this is that at the time of reaching the target weight in the target container, additional substance has already left the dispensing head and is for example in free fall between the dosage-dispensing device and the target container, affecting the weighing result only after the additional substance is at rest on the bottom of the target container or on top of the previously dispensed substance.
The width of the tolerance band of the dosage-dispensing device, expressed by the scatter range of the achievable target weights is affected in particular by the response time of the balance. Due to the relatively late arrival of the weighing signal in the control- and regulation unit of the dosage-dispensing device, too much substance has already been dispensed in the meantime. In many dosage-dispensing devices which like the device disclosed in WO 2004/042334 A1 provide a continuous delivery of substance from the beginning to the end of the dosage-dispensing process, the mass flow or volume flow of the material to be dispensed is slowed down towards the end of the dosage-dispensing process by a dispensing program which is executed in the control- and regulation unit, in order to take the response time of the balance or of the electronic weighing system into account. However, as a result the dosage-dispensing process can take longer depending on the precision required so that for a specified narrow tolerance of the target weight or target quantity the dosage-dispensing process can take a very long time.
The dispensing program determines the variable degree to which an outlet opening at the dispensing head is opened and closed over the time of the dosage-dispensing process.
Unlike liquids, whose flow behavior remains approximately constant under given ambient conditions such as pressure and temperature, the flow properties of substances in powder- or paste form is affected by a considerably larger number of influence factors. The same powder can exhibit totally different flow behaviors simply due to different bulk densities. For example compressed wheat flour behaves in a fundamentally different way compared to loosely aggregated flour.
The flow properties of a substance in a dispensing process can also change strongly as a result of a change in the relative humidity of the ambient atmosphere or of the substance itself.
Also, particle size or grain size and the grain size distribution as well as the shape of the individual particles of a substance in a dosage-dispensing process can have a decisive influence on the flow behavior. All of these influence factors can make it nearly impossible to use one and the same dosage-dispensing program for the entire range of known powders with a uniform tolerance bandwidth.